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VOICE ONE:
This is Steve Ember.
VOICE TWO:
And this is Bob Doughty with theVOA Special English program EXPLORATIONS. Today, we tell about howpeople learned an important piece of information necessary forsafely sailing on the oceans. It is called longitude.
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VOICE ONE:
On a foggy October night in 1707, four English navy ships hitrocks in the Atlantic Ocean and sank. Two-thousand men drowned. Theships had been sailing in the thick fog for twelve days. There wasno sure way to know where they were. The commander of the ships hadbeen worried that they could hit rocks if they were not careful. Heasked his navigators for their opinion on their location in theocean.
The navigators did not really know. They told the commander theythought they were west of a small island near the coast ofnorthwestern France.
They were wrong. Instead, they sailed onto rocks near a smallgroup of islands southwest of England’s Atlantic coast. Thenavigators’ lack of knowledge led to the loss of four ships andtwo-thousand lives.
VOICE TWO:
When people began sailing out of sight of land, sailors did notknow how to tell where they were on the open sea. Land travelers canlook at a mountain,or a river, or an object that shows them wherethey are in relation to where they came from. On the ocean, however,there is no sign to tell a sailor where he is.
The most important device forknowing directions on the ocean is a compass. A compass is a devicecontaining a metal object that points toward the magnetic northpole. This shows navigators the direction of north, and thereforealso south, east, and west. But sailors need more information tosail safely on the open sea.
VOICE ONE:
Most maps of the world show lines that are not on the Earth’ssurface. One line is the equator. It is an imaginary line around thewidest part of the Earth. There are similar lines both north andsouth of the equator. These circles become smaller and smallertoward the north pole and the south pole.
These lines, or circles, are parallel – meaning that they areequally distant from each other at any point around the world. Theselines show what is called latitude.
A navigator can know the latitude of his ship by observing thelocation of stars, where the sun rises in the morning and sets inthe evening, and what time of year it is. With this information heknows where his ship is in relation to the north or south pole andthe equator.
VOICE TWO:
Still, there is one more important piece of information necessaryfor safely sailing the oceans. For many centuries, scientists,astronomers and inventors searched for a way to tell longitude. Thelines of longitude go the other way from latitude lines. Theystretch from the north pole to the south pole, and back again ingreat circles of the same size. All of the lines of longitude meetat the top and bottom of the world.
In her book, Longitude, writerDava Sobel tells the story about longitude and how the problem ofknowing it was solved.
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VOICE ONE:
For centuries, the great scientists of the world struggled todevelop a way to learn longitude. To learn longitude at any placerequires knowledge about time. A navigator needs to know what timeit is on his ship and also the time at another place of knownlongitude – at the very same moment.
The Earth takes twenty-four hours to complete one full turn orrevolution of three-hundred-sixty degrees. One hour marks onetwenty-fourth of a turn, or fifteen degrees. So each hour’s timedifference between the ship and the starting point marks a ship’sprogress of fifteen degrees of longitude to the east or west. Thosefifteen degrees of longitude mark a distance traveled.
At the equator, where the Earth is widest, fifteen degreesstretches about one-thousand-six-hundred kilometers. North or southof that line, however, the distance value of each degree decreases.One degree of longitude equals four minutes of time all around theworld. But in measuring distance, one degree shrinks from aboutone-hundred-nine kilometers at the Equator to nothing at the northand south poles.
VOICE TWO:
For many centuries, navigators hoped they could find longitude byobserving the movement of stars at night. During the day, the sunprovided information about the time on a ship, and its direction.However, it did not provide necessary information about the timesomewhere else.
In the Sixteenth century, one astronomer suggested thatnavigators could observe the moon as it passed in front of differentknown stars to tell longitude. But, there was not enough informationabout the stars to use this method effectively. Astronomers couldnot tell exactly where the moon would be from one night or day tothe next.
Yet it seemed to those seeking to solve the longitude problemthat the only solution was in the moon and stars.
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VOICE ONE:
During the Seventeenth Century, English astronomers began a majoreffort to map the stars and their relationship to the moon as itpassed across the sky. Royal Astronomer John Flamsteed worked atthis task for forty years. The next Royal Astronomer, Edmund Halley,spent another forty years gathering information about the moon’sorbit.
After many years of gathering the necessary information, itbecame possible to learn longitude by observing the stars and themoon. In Seventeen-Sixty- Six, Royal Astronomer Nevil Maskelynepublished the Nautical Almanac and Astronomical Ephemeris.
It contained all the necessary information about the moon andstars that sailors would need to help them learn their longitude.
This new method was not simple. A navigator had to use complexobserving instruments to note the position of the moon and stars.Then he had to seek the correct information in the Nautical Almanacabout the moon and stars at that time of night or day. The finalstep in the process was to take the mathematical information fromthe book, link it to the current information and solve the resultingproblem. This took an average of four hours to do.
VOICE TWO:
While scientists were studying the stars and moon to solve thelongitude problem, a man named John Harrison was working on anotherproject. He was trying to build a clock that would help sailorslearn longitude. His task also was difficult and complex. MisterHarrison had to develop a clock that was not affected by themovement of a ship on the ocean or changes in temperature oratmospheric pressure.
He began developing his clock in 1730. It took five years tocomplete. The complex device weighed thirty-four kilograms. Severalyears later, Mister Harrison built a second clock. It was smaller,but weighed more than the first. Mister Harrison was not satisfiedand began working on yet another device.
Twenty years later, he completed a device that was smaller thanthe first two, and weighed less. But, still Mister Harrison was notsatisfied.
Two years later, in 1757, he produced a small clock that he couldhold in his hand. The clock could tell the correct time in twoplaces, meeting the requirements for learning longitude on the sea.
VOICE ONE:
For many years after Mister Harrison’s work was completed, theidea of using a clock to learn longitude was rejected. However, thatopinion changed when manufacturers learned how to make better andless costly versions of Mister Harrison’s clocks. The clocks becameknown as chronometers. By 1815, five-thousand chronometers were inuse on ships sailing the world’s oceans. The complex documents andmathematical work were no longer necessary. Almost any sailor couldtell what his longitude was by simply looking at a clock. The worldhad changed.
VOICE TWO:
John Harrison’s clocks can be seen today at the Old RoyalObservatory in Greenwich, England. The first three are stilloperating, showing the correct time. To look at them is to see thesimple solution to a problem that worried people for many centuries.Today, the solution to the problem is so common that it is difficultto understand that there was a problem at all.
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VOICE ONE:
This program was written by Oliver Chanler and produced by PaulThompson. Our studio engineer was Al Alaby. This is Steve Ember.
VOICE TWO:
And this is Bob Doughty. Join us again next week forEXPLORATIONS, a program in Special English on the Voice of America.